Circuit interrupters, such as Ground Fault Circuit Interrupters (GFCIs) are employed in a variety of environments to interrupt power provided to various loads, such as household appliances, electrical products and branch circuits. Typically, circuit interrupters use a trip mechanism to mechanically break an electrical connection between an input and one or more output conductors, when, for example, a ground fault is sensed by a sensing circuit incorporated within the circuit interrupter.
The sensing circuit can incorporate a high turn ratio differential transformer connected to detect the difference of current in the neutral and phase wires of an electrical distribution system. The secondary coil of the differential transformer can be connected to an input of a high gain operational amplifier (OA) on a chip having an integrated circuit for detecting ground faults. The high gain OA provides an amplified signal which is proportional to the differential of the neutral and phase current. This amplified signal is used to charge a capacitor. Under a ground fault condition (where a GFCI is used), the amplified signal will quickly charge the capacitor in excess of a threshold value, whereupon a relay trigger is energized, causing a set of contacts to open to disconnect the load from the line.
As noted, circuit interrupters are designed to trip (e.g. interrupt current) when a fault is present. It can be advantageous to interrupt a larger fault current sooner since a larger ground fault current presents a greater shock risk to personnel. Underwriters Laboratories (UL) Standard 943 requires, for a Class “A” GFCI, that the GFCI must trip when a ground fault is sensed within a certain period of time, depending on the magnitude of the ground fault current (e.g. trip-time response). The trip-time response is dictated by the equation:
T=(20/I)1.43 where “T” is time measured in seconds, and “I” is current measured in milliamps (mA).
For example, with a ground fault of 5 mA, the GFCI would be required to trip within about 7¼ seconds.
Circuit interrupters can be calibrated to trip within a time inversely proportional to the magnitude of the fault current in accordance with the above equation. Once calibrated, however, the trip-time response can be subject to one or more shifts caused by a variety of environmental and operational factors. Examples of such environmental factors include temperature fluctuations, while examples of operational factors include input voltage and load. As will be appreciated, shifts of the trip-time response are undesirable because they can cause the circuit interrupter to have a trip-time response that is either slower or faster than a predetermined trip-time response. For the example above, a trip-time response shift may result in a 5 mA ground fault not tripping the circuit interrupter until after the about 7¼ second period has elapsed.
In view of this, it would be desirable to provide a self-calibrating circuit interrupter that automatically and/or periodically adjusts the trip-time response to counteract shifts caused by input voltage, load, temperature and/or other factors.